The present invention relates to an optical tracking servo apparatus, and particularly relates to an optical tracking servo apparatus constructed so as to perform a tracking control of a light beam irradiated on an information recording surface on a predetermined track of a disc shaped recording medium, by detecting a reflected light from the information recording surface of the disc shaped recording medium.
In the conventional optical magnetic disc apparatus, a light beam is irradiated from a laser diode, passed through a mirror and an objective lens, and is thereby irradiated onto an information recording surface (hereinafter referred to as a recording surface) on a predetermined track of a disc shaped recording medium (hereinafter referred to as a disc). Thereafter, a reflected light of the irradiated light beam is reflected from the recording surface and onto a photodetector. The photodetector may for example have two photodiodes unitarily formed adjacent to one another. The photodetector obtains a tracking error signal based on respective portions of the reflected light incident on the respective photodiodes in accordance with the known push-pull method.
With the optical magnetic disc apparatus, it is necessary to perform a tracking control in order to accurately irradiate the light beam onto a predetermined track of the recording surface of the disc. There are two methods by which such a control can be performed. In a first method, the tracking control is performed by driving the objective lens in the radial direction of the disc in accordance with the tracking error signal. In a second method, the tracking control is performed by driving the semi-transparent mirror which directs the light beam to the objective lens by reflection of the light beam from the laser diode, in accordance with the tracking error signal. In both of the first and second methods, a magnetic coil is conventionally used as an actuator for driving the objective lens and the semi-transparent mirror respectively by electromagnetic force generated in the magnetic coil.
However, with the conventional tracking servo apparatus which uses the magnetic coil as the actuator, there are problems in that the size as well as consumption power of the magnetic coil constituting the actuator is large. Further a magnetic coil for recording is used by the optical magnetic disc apparatus to apply a magnetic field to magnetize the recording surface of the magnetic disc in order to record recording information in digital form. However, there is a problem that when a current is passed through the magnetic coil used as the actuator a magnetic field generated therefrom interferes with the magnetic field applied to the recording surface by the magnetic coil for recording, and therefore a magnetic noise results in the information recorded on the magnetic disc.
Further, a light beam focusing assembly has the objective lens, the magnetic coil as the actuator, and the mirror, and by moving the light beam focusing assembly in the radial direction of the disc, the light beam is irradiated onto a predetermined track. There is a demand to shorten an access time in order to achieve an increased speed by which the focusing assembly is moved for tracking the light beam onto the predetermined track during recording and reproduction, by decreasing the weight and size of the light beam focusing assembly. However, with the conventional tracking servo apparatus, it is difficult to decrease the size of the magnetic coil constituting the actuator within the focusing assembly, and therefore it is difficult to increase the speed of access to a predetermined track during recording and reproduction.